A. Field
Embodiments described herein relate generally to an X-ray computer tomography (CT) apparatus and a method for detecting a magnetic pole position thereof, and more particularly, to an X-ray CT apparatus having a direct drive motor driving mechanism for rotating a gantry frame and a method for detecting a magnetic pole position of the X-ray CT apparatus at a starting-up time of the gantry frame.
B. Background
An X-ray CT apparatus, widely used in medical and industrial fields, acquires projection data for reconstructing tomography images of an object by irradiating radiation beams around the object. More particularly, the X-ray CT apparatus acquires projection data by rotating an X-ray tube and an X-ray detection unit around the object in one direction, where the X-ray tube and the X-ray detection unit are situated on a ring-shaped rotation frame (gantry rotation unit) in a gantry so as to face each other.
Recently, a high-speed and a high-functioning X-ray CT apparatus has been developed, wherein the rotation frame is rotated at a high speed to acquire projection data at a high speed. In order to rotate the rotation frame at a high speed, the power supply and the signal transmissions and receptions between the rotation frame and a fixed gantry supporting the rotation frame are performed through a slip ring or an optical transmission and the rotation power of a motor situated on the fixed gantry is transferred to the rotation frame through the gears and belts of the motor. However, such a rotation driving power transferring system generates relatively large mechanical vibrations and operation sounds that may cause discomfort to a patient or an operator.
Recently, a high speed X-ray CT apparatus was proposed wherein quiet sound imaging is achieved without using gears and belts. Instead, the apparatus uses a direct drive motor (hereinafter, simply referred to as “DD motor”) driving system that transfers the rotation power to the rotation frame by non-contacting technology.
In the DD motor driving system, the non-contacting rotation power transmission is performed between the windings on the rotation frame and the magnets on the gantry fixing unit. More specifically, the rotation frame is directly rotationally driven as a non-contacting rotor member of a repulsion motor by repulsions between the magnetic fluxes generated by supplying a current to the windings of the rotation frame and the magnetic fluxes from the magnets on the gantry fixing unit. The repulsion DD motor driving circuit may also be constructed by situating the windings on the gantry fixing unit so as to act repulsively to the magnets on the rotation frame.
At a start-up time of a DD motor, where the rotation frame is constructed as a non-contacting rotor member, a rotation direction of the rotation frame and switching timings of the magnetic poles can be determined by controlling pulse signals supplied to the windings. The pulse signals are supplied in accordance with both a center-to-center distance between a center position of the winding and a center position of the fixed magnet, and the magnetic pole (N-pole, S-pole) of the rotor magnet. Usually, the magnetic pole position at the starting-up time of the DD motor is determined by exciting the gantry rotation frame—e.g., by supplying currents to the windings of the rotation unit during a very short time period—and estimating the magnetic pole position at starting-up time of the DD motor based on the time period between a resulting vibration start-up time and a vibration stop time.
In the conventionally proposed quiet X-ray CT apparatus of the DD motor driving system, the fixed gantry supports the rotation frame through ball bearings, and the fixed gantry supplies rotation power to the rotation frame through a slip ring between the rotation frame and the fixed gantry. As a result, when the DD motor is started up, the slip ring and the ball-bearings create friction resistances of the rotation frame against the fixed gantry. Thus, when the rotation frame is excited during a very short time, the vibration of the rotation frame converges, or gradually focuses, and stops after a prescribed time period due to the friction resistances. Accordingly, it is possible to estimate the magnetic pole position based on the time period between the start of vibration and the end of the vibration.
Nevertheless, a more silent imaging of the X-ray CT apparatus has persistently been required. One approach replaces the power supply from the fixed gantry to the rotation frame through a slip ring with a non-contacting power supply and the support of the rotation frame on the fixed gantry through ball-bearings with an air bearing support.
However, supporting the rotation frame by the fixed gantry using air bearings makes it difficult to easily estimate the magnetic pole position by exciting the rotation frame during a short time at a start-up of the DD motor because the vibration of the rotation frame cannot be easily converged and stopped in a short time due to the much lower friction resistances of the rotation frame against the fixed gantry than in contacting supports. Thus, it becomes difficult to easily estimate the magnetic pole position at the starting-up time of the DD motor in a short time. This generates a drawback that it takes a long time until acquisition of projection data can be started.